This is the blog of the Irish Climate Analysis and Research Units hosted by the Department of Geography at Maynooth University. It is primarily used to highlight newly published research and activities that may be of general interest.

Wednesday, January 10, 2018

The Atlantic Overturning Circulation and Ireland

Palm trees in Sligo

Figure 1: Palm trees grow in Mullaghmore in Sligo
(left), which is as far north as glaciated Grytviken, South Georgia (right) is
south, where glaciers grow.[image modified from:Wikipedia]

Sometimes it’s easy to forget how far north Ireland is. Palm
trees grow in Sligo on the northwest Atlantic coast whereas glaciers---though in retreat ---overshadow
Grytviken, South Georgia (Fig. 1). Grytviken is as far south as Sligo is north,
so why the big difference?

Frequently people will cite the Gulf Stream
as the reason for Ireland’s mild climate. This is an idea dating back to
Matthew Fontaine Maury
who coined the idea that the Gulf Stream was responsible for Europe’s mild
climate in 1855. But the Gulf Stream itself is not inherently unique. It is an
ocean current, called a ‘Western Boundary Current’ due to its location in the
western North Atlantic, analogues of which exist in every major ocean basin. In
the North Pacific, the Kuroshio
is a western boundary current just as vigorous as the Gulf Stream. But when we
look at surface temperatures relative to relative to the average for that
latitude (Fig. 2), the Atlantic does stand out as anomalously warm.

Atlantic Exceptionalism

The ocean process
that drives the anomalous warmth in the Atlantic is the Atlantic Overturning
Circulation. This is a system of currents that carries warm water northwards, through
the Gulf Stream and its extended current, the North Atlantic Current. This
water is warm and salty. As it loses heat, through a variety of processes, it
forms deep, cold water that returns southwards. This exchange of warm and cold
water leads to the largest movement of heat by any ocean. This heat is then
released from the Atlantic and is carried by the prevailing winds over
northwest Europe. This combination of ocean and wind is the reason for the
exceptionally mild climate of northwest Europe.

Figure 3: Dublin is
around 9C warmer than St Johns in Newfoundland and 4C warmer than the similar
maritime climate of Seattle on the Pacific Northwest.[image from McCarthy et al., 2015, Weather]

The combination of winds and ocean is
important and leads to a subtlety in discussing this Atlantic Exceptionalism.
It might be tempting to look across from the cliffs of Moher in winter to the
frigid Labrador coast of Canada and
ascribe the almost 10 degrees C difference in temperature to the Atlantic
Overturning. However, much of this temperature difference is due to this coast
being downwind of the frigid Canadian landmass whereas Ireland is downwind of
the ocean. A better comparison to reveal the impact of the Atlantic overturning
on climate is to compare Ireland to similar maritime climates of the Pacific
northwest. Dublin is 4C warmer than Seattle in winter and this difference can
be mainly ascribed to the influence of the Atlantic Overturning Circulation.
While not as dramatic a comparison as with the coast of Labrador, a mean
difference of 4C is approximately the difference in temperature between the
climate of Ireland and the climate of Northern Portugal.

Threats to the Overturning Circulation

The Atlantic Overturning has changed in
the past, with large fluctuations in climate in the last glacial period
associated with collapsing and recovering of the Overturning circulation, and
it is predicted to decline in the future---the Intergovernmental Panel for
Climate Change ranking as 'very likely' that the Overturning Circulation will
weaken by 30% in the coming century. If the overturning declines, this
will have a serious impact on Irish climate. Evidence for a declining
overturning is already mounting. Temperatures globally continue to rise due to
anthropogenic global warming but, in the North Atlantic, a cold spot appears
where a small temperature fall is occurring.

Here at ICARUS, work is ongoing to
understand the ongoing changes in the Atlantic, including understanding the
drivers and the impacts that has on Ireland, northwestern Europe and beyond.

Figure 4: Global
surface temperatures show a general increase due to anthropogenic climate
change. A blue area in the North Atlantic shows where temperatures have not
risen and could be indicative of a slowdown in the Atlantic Overturning
Circulation.

Hi Connie. In terms of mean climate, we could expect a drop in temperatures relative to global climate of about 5ºC with the total collapse of the Overturning Circulation. In terms of present day climate, that would make Ireland's climate closer to that of Iceland.

In terms of weather, one of the likely effects would be an increase in storminess. The jet stream that is responsible for much of the weather directed towards Ireland is influenced by the temperature of the ocean below. A slowdown in the overturning would cause the northern part of the Atlantic to cool substantially. Indications are that this could increase storminess in Ireland. Indeed, the winter of 2013/14 was the stormiest on record in Ireland and this was a time when the overturning was weak.

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